When should I reboot?

I reboot when I yum update to a new kernel or systemd, which seems to come
out about once a month. Should I do it for this week's glibc? Is that
"core" enough to justify a reboot or should I wait for the next kernel
update? I know the glibc update was mainly to handle the new Japanese
calendar, so that shouldn't affect my usage. So my question is more about
how shared libraries work and whether anything bad would happen with
different forks of running services (mainly the mail suite with dovecot and
the various content scanners launched by sendmail) running different
versions of the library based on when they were started.